cifs: fix regression when mounting shares with prefix paths

The commit 315db9a ("cifs: fix leak in cifs_smb3_do_mount() ctx")
revealed an existing bug when mounting shares that contain a prefix
path or DFS links.

cifs_setup_volume_info() requires the @devname to contain the full
path (UNC + prefix) to update the fs context with the new UNC and
prepath values, however we were passing only the UNC
path (old_ctx->UNC) in @device thus discarding any prefix paths.

Instead of concatenating both old_ctx->{UNC,prepath} and pass it in
@devname, just keep the dup'ed values of UNC and prepath in
cifs_sb->ctx after calling smb3_fs_context_dup(), and fix
smb3_parse_devname() to correctly parse and not leak the new UNC and
prefix paths.

Cc: <stable@vger.kernel.org> # v5.11+
Fixes: 315db9a ("cifs: fix leak in cifs_smb3_do_mount() ctx")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Acked-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Paulo Alcantara

fs/cifs/cifsfs.c

@@ -863,13 +863,7 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 		goto out;
 	}
 
-	/* cifs_setup_volume_info->smb3_parse_devname() redups UNC & prepath */
-	kfree(cifs_sb->ctx->UNC);
-	cifs_sb->ctx->UNC = NULL;
-	kfree(cifs_sb->ctx->prepath);
-	cifs_sb->ctx->prepath = NULL;
-
-	rc = cifs_setup_volume_info(cifs_sb->ctx, NULL, old_ctx->UNC);
+	rc = cifs_setup_volume_info(cifs_sb->ctx, NULL, NULL);
 	if (rc) {
 		root = ERR_PTR(rc);
 		goto out;

fs/cifs/connect.c

@@ -3149,17 +3149,29 @@ static int do_dfs_failover(const char *path, const char *full_path, struct cifs_
 int
 cifs_setup_volume_info(struct smb3_fs_context *ctx, const char *mntopts, const char *devname)
 {
-	int rc = 0;
+	int rc;
 
-	smb3_parse_devname(devname, ctx);
+	if (devname) {
+		cifs_dbg(FYI, "%s: devname=%s\n", __func__, devname);
+		rc = smb3_parse_devname(devname, ctx);
+		if (rc) {
+			cifs_dbg(VFS, "%s: failed to parse %s: %d\n", __func__, devname, rc);
+			return rc;
+		}
+	}
 
 	if (mntopts) {
 		char *ip;
 
-		cifs_dbg(FYI, "%s: mntopts=%s\n", __func__, mntopts);
 		rc = smb3_parse_opt(mntopts, "ip", &ip);
-		if (!rc && !cifs_convert_address((struct sockaddr *)&ctx->dstaddr, ip,
-						 strlen(ip))) {
+		if (rc) {
+			cifs_dbg(VFS, "%s: failed to parse ip options: %d\n", __func__, rc);
+			return rc;
+		}
+
+		rc = cifs_convert_address((struct sockaddr *)&ctx->dstaddr, ip, strlen(ip));
+		kfree(ip);
+		if (!rc) {
 			cifs_dbg(VFS, "%s: failed to convert ip address\n", __func__);
 			return -EINVAL;
 		}
@@ -3179,7 +3191,7 @@ cifs_setup_volume_info(struct smb3_fs_context *ctx, const char *mntopts, const c
 		return -EINVAL;
 	}
 
-	return rc;
+	return 0;
 }
 
 static int

fs/cifs/fs_context.c

@@ -476,6 +476,7 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 
 	/* move "pos" up to delimiter or NULL */
 	pos += len;
+	kfree(ctx->UNC);
 	ctx->UNC = kstrndup(devname, pos - devname, GFP_KERNEL);
 	if (!ctx->UNC)
 		return -ENOMEM;
@@ -486,6 +487,9 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 	if (*pos == '/' || *pos == '\\')
 		pos++;
 
+	kfree(ctx->prepath);
+	ctx->prepath = NULL;
+
 	/* If pos is NULL then no prepath */
 	if (!*pos)
 		return 0;